"UBS has discovered a loss due to unauthorized trading by a trader in its Investment Bank. The matter is still being investigated, but UBS's current estimate of the loss on the trades is in the range of USD 2 billion. It is possible that this could lead UBS to report a loss for the third quarter of 2011. No client positions were affected."

The UBS press release also gave a bit more detail about what happened:

"The loss resulted from unauthorized speculative trading in various S&P 500, DAX, and EuroStoxx index futures over the last three months. The positions taken were within the normal business flow of a large global equity trading house as part of a properly hedged portfolio. However, the true magnitude of the risk exposure was distorted because the positions had been offset in our systems with fictitious, forward-settling, cash ETF positions, allegedly executed by the trader. These fictitious trades concealed the fact that the index futures trades violated UBS's risk limits."

"Following inquiries directed to him by UBS control functions that were reviewing his positions, the trader revealed his unauthorized activity on September 14, 2011."